Control circuit having adaptive blanking time and method for providing the same

ABSTRACT

A control circuit can include: a blanking time control circuit configured to generate a blanking control signal according to a peak current of a main switch of a power stage circuit of a flyback converter; a sampling time control circuit configured to generate a sampling time control signal according to the blanking control signal and a feedback voltage signal; and a voltage detection circuit configured to receive the feedback voltage signal and the sampling time control signal, and to determine the time of detecting the feedback voltage signal according to the sampling time control signal to obtain a detection signal for controlling the main switch, where the voltage detection circuit stops detecting the feedback voltage signal when the blanking control signal is active, and the period during which the blanking control signal is active is adjustable along with the peak current.

RELATED APPLICATIONS

This application claims the benefit of Chinese Patent Application No.201610048123.1, filed on Jan. 22, 2016, which is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

The present invention generally relates to the field of powerelectronics, and more particularly to control circuits having anadaptive blanking time and associated control methods.

BACKGROUND

Driving the gate of a main power switch in a switching power supply mayhave a great impact on power performance. Serious electromagneticinterference (EMI) may result because of relatively large di/dt anddv/dt of the main power switch if the driving capacity is too strong(e.g., the driving current is too large). Also, switching losses mayincrease because of slow switching speeds due to relatively largeswitching delays of the main power switch if the driving capacity is tooweak (e.g., the driving current is too small).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of an example primary controlledflyback converter.

FIG. 2 is a schematic block diagram of an example control circuit havingan adaptive blanking time, in accordance with embodiments of the presentinvention.

FIG. 3 is a schematic block diagram of an example sampling time controlcircuit of the control circuit FIG. 2, in accordance with embodiments ofthe present invention.

FIG. 4 is a more detailed schematic block diagram of the examplesampling time control circuit of FIG. 3, in accordance with embodimentsof the present invention.

FIG. 5 is a schematic block diagram of an example blanking time controlcircuit of the control circuit of FIG. 2, in accordance with embodimentsof the present invention.

FIG. 6 is a waveform diagram showing a first example operation of thecircuit of FIG. 2, in accordance with embodiments of the presentinvention.

FIG. 7 is a waveform diagram showing a second example operation of thecircuit of FIG. 2, in accordance with embodiments of the presentinvention.

DETAILED DESCRIPTION

Reference may now be made in detail to particular embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. While the invention may be described in conjunction with thepreferred embodiments, it may be understood that they are not intendedto limit the invention to these embodiments. On the contrary, theinvention is intended to cover alternatives, modifications andequivalents that may be included within the spirit and scope of theinvention as defined by the appended claims. Furthermore, in thefollowing detailed description of the present invention, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. However, it may be readilyapparent to one skilled in the art that the present invention may bepracticed without these specific details. In other instances, well-knownmethods, procedures, processes, components, structures, and circuitshave not been described in detail so as not to unnecessarily obscureaspects of the present invention.

Referring now to FIG. 1, shown is a schematic block diagram of anexample primary controlled flyback converter. In this particularexample, an auxiliary winding at the primary side can be coupled to asecondary winding of a transformer to obtain a feedback signal thatrepresents the output voltage. This is because the output voltage isproportional to a voltage across the auxiliary winding when afreewheeling current through a secondary winding diode D₁ approacheszero. Thus, the output voltage can be well controlled by sampling thevoltage across the auxiliary winding as the feedback signal of theoutput voltage. When the secondary current is zero, the voltage acrossthe auxiliary winding may no longer be clamped by the output voltage,and may vibrate and have a relatively large voltage drop. Thus, thesampling time may usually be determined by sampling the voltage drop.

However, due to the presence of the leakage-inductance of thetransformer and the reverse recovery diode of an auxiliary power supplyand other effects, the voltage across the auxiliary winding may vibrateheavily after a main switch at the primary side is turned off, which canresult in “mis-detection” of the feedback voltage. Therefore, a blankingtime can be set after the main switch at the primary side is turned off,and sampling can begin by escaping the vibration time period. However,it may be difficult to appropriately set the blanking time because thevibration time period is not a fixed time period, and may change alongwith the peak current of the main switch at the primary side.

In one approach, a relatively longer blanking time can be set in orderto avoid mis-detection when the peak current of the main switch at theprimary side is relatively large. In another approach, an adaptiveblanking time can be set according to the load condition. In the firstapproach, the blanking time can be relatively long even when the peakcurrent is relatively small. This can mean a relatively large phantomload can exist in order to meet requirements of the blanking time in ano-load condition. Thus, the power losses in the no-load condition maybe relatively large, which can adversely affect energy-saving demands.In the second approach, the load condition may not precisely indicatethe peak current in some applications. For example, in a constant ontime control power factor correction (PFC) circuit, the peak current mayincrease along with the input voltage while the load remains unchanged.Thus, the detection may not be correct if the blanking time is set basedon the load condition.

In one embodiment, a control circuit can include: (i) a blanking timecontrol circuit configured to generate a blanking control signalaccording to a peak current of a main switch of a power stage circuit ofa flyback converter, where the power stage circuit comprises atransformer having primary and secondary windings, the main switchcoupled to the primary winding, and a rectifying diode coupled to thesecondary winding; (ii) a sampling time control circuit configured togenerate a sampling time control signal according to the blankingcontrol signal and a feedback voltage signal, where the feedback voltagesignal represents an output voltage signal at the secondary side of thetransformer; and (iii) a voltage detection circuit configured to receivethe feedback voltage signal and the sampling time control signal, and todetermine the time of detecting the feedback voltage signal according tothe sampling time control signal in order to obtain a detection signalfor controlling the main switch, where the voltage detection circuitstops detecting the feedback voltage signal when the blanking controlsignal is active, and the period during which the blanking controlsignal is active is adjustable along with the peak current.

Referring now to FIG. 2, shown is a schematic block diagram of anexample control circuit having an adaptive blanking time, in accordancewith embodiments of the present invention. In this particular example,the flyback converter can receive input voltage signal V_(IN), and maygenerate a substantially stable output voltage Vo for a load. Forexample, the flyback converter can include transformer T having primarywinding Np and secondary winding Ns, main switch S₁ coupled to theprimary winding, and a rectifying device coupled to a secondary winding.In this example, diode D₁ can be used as the rectifying device with theanode connected to the secondary winding and the cathode connected tothe output terminal.

The control circuit can include output voltage feedback circuit 201,blanking time control circuit 202, sampling time control circuit 203,voltage detection circuit 204, and driving control circuit 205. Outputvoltage feedback circuit 201 can generate feedback voltage signalV_(ZCS) that represents the output voltage, and may include auxiliarywinding N_(A), and division resistors R1 and R2. In this example,blanking time control circuit 202 can generate blanking control signalT_(blanking) according to peak current information of the main switch.Sampling time control circuit 203 can generate sampling time controlsignal Tsample according to blanking control signal T_(blanking) andfeedback voltage signal V_(ZCS). Voltage detection circuit 204 canreceive feedback voltage signal V_(ZCS) and sampling time control signalTsample, and may determine the time for detecting the feedback voltageaccording to the sampling time control signal, in order to obtaindetection signal Vd. Driving control circuit 205 can control main switchS₁ by switching control signal VGate according to detection signal Vd.

Referring forward to FIG. 5, shown is a schematic block diagram of anexample blanking time control circuit of the control circuit of FIG. 2,in accordance with embodiments of the present invention. In thisparticular example, blanking time control circuit 202 can include acharge and discharge circuit and a comparison circuit. The charge anddischarge circuit can include current source I1 being configured togenerate current signal I1 according to the peak current information ofthe main switch, switch S2, and capacitor C2 coupled in parallel toswitch S2.

In this example, current signal I1 can be inversely proportional to peakcurrent V_(IPK). Current source I1 can be configured to charge capacitorC2, and a voltage across capacitor C2 may be configured as voltagesignal Vs2. The comparison circuit can include a comparator having aninverting input terminal that receives voltage signal Vs2, and anon-inverting input terminal that receives reference signal Vref. Thecomparator can compare voltage signal Vs2 against reference signal Vref,and may generate blanking control signal T_(blanking). For exampleswitch S2 can be controlled by the switching control signal of the mainswitch (e.g., switch S₁). In this example, the blanking time controlcircuit may also include current source I2 (e.g., a fixed current sourceused to charge capacitor C2) coupled in parallel to current source I1.Current source I2 can be used to ensure that the blanking time has amaximum value, to ensure that sampling will not be carried out duringthe vibration state of the feedback voltage signal.

Referring now to FIGS. 3 and 4, shown are schematic block diagrams of anexample sampling time control circuit of the control circuit FIG. 2, inaccordance with embodiments of the present invention. In this example,the blanking circuit can include resistor R11 coupled to switch S1.Resistor R11 may have a first terminal configured to receive feedbackvoltage signal V_(ZCS), and a second terminal coupled to the holdingcircuit. Also, switch S1 can be controlled by blanking control signalT_(blanking).

The holding circuit can include capacitor C1 having a first terminalcoupled to the second terminal of resistor R11, and a second terminalthat is grounded. When blanking control signal T_(blanking) is active,switch S1 can be turned on, and feedback voltage signal V_(ZCS) may bedirectly transmitted to capacitor C1. When the blanking control signalis inactive, the blanking circuit may transmit feedback voltage signalV_(ZCS) to capacitor C1 through resistor R11. The voltage acrosscapacitor C1 may be configured as holding signal V2, and resistor R11and capacitor C1 can form a delay circuit.

A voltage difference circuit can include a voltage source having aninverting terminal that receives the feedback voltage signal, and anon-inverting terminal coupled to a comparison circuit. For example, thevoltage source may have a relatively small value (e.g., about 100 mV).Also, voltage signal V1 that is greater than feedback voltage signalV_(ZCS) can be obtained through the voltage difference circuit. Thecomparison circuit can include a comparator having a non-inverting inputterminal that receives holding signal V2, an inverting input terminalthat receives voltage signal V1, and an output terminal that generatessampling time control signal T_(sample). When holding signal V2 isgreater than voltage signal V1, the sampling time control signal may gohigh.

The period during which blanking control signal T_(blanking) is in anactive state can be referred as the blanking time. During the blankingtime, switch S1 may be turned on, and the comparator may not changestates. Thus, the feedback voltage signal will not be detected even asfeedback voltage signal V_(ZCS) fluctuates, so as to achieve theblanking function. While this particular sampling time control circuitis only one example implementation, those skilled in the art willrecognize that other sampling time control circuits can also beutilized.

In one embodiment, a method can include: (i) generating a blankingcontrol signal according to a peak current of a main switch of a powerstage circuit of a flyback converter, where the power stage circuitcomprises a transformer having primary and secondary windings, the mainswitch coupled to the primary winding, and a rectifying diode coupled tothe secondary winding; (ii) generating a sampling time control signalaccording to the blanking control signal and a feedback voltage signal;(iii) receiving the feedback voltage signal and the sampling timecontrol signal, and determining the time of detecting the feedbackvoltage signal according to the sampling time control signal in order toobtain a detection signal for controlling the main switch; and (iv)using the feedback voltage signal to represent an output voltage at thesecondary side of the transformer, where the voltage detection circuitstops detecting the feedback voltage signal when the blanking controlsignal is active, and the period during which the blanking controlsignal is active is adjustable along with the peak current.

Referring now to FIG. 6, shown is a waveform diagram of a first exampleoperation of the circuit of FIG. 2, in accordance with embodiments ofthe present invention. When the system is in a discontinuous operatingmode under a light-load condition, peak current I_(PP) of the mainswitch at the primary side may be relatively small. As shown, “IS” is adischarge current of the secondary diode. At time t1, the main switch S1is turned off, and the feedback voltage signal of the secondary windingcan fluctuate. In this case, the discharge time (e.g., T_(DIS)) of thesecondary side may be relatively short. Thus, the time period (e.g.,time period T1 in FIG. 6) during which the blanking control signal isactive may be relatively short, such that the voltage detecting circuitcan quickly detect feedback voltage signal V_(ZCS).

Referring now to FIG. 7, shown is a waveform diagram of a second exampleoperation of the circuit of FIG. 2, in accordance with embodiments ofthe present invention. When the system is in a quasi-resonant operatingmode under a heavy-load condition, the peak current I_(PP) of the mainswitch at the primary side can be relatively large. As shown, “IS” isthe discharge current of the secondary diode. At time t1, main switch S1can be turned off, and the feedback voltage signal of the secondarywinding can fluctuate. In this case, the discharge time (e.g., T_(DIS))of the secondary side may be relatively long. Thus, the time period(e.g., time period T2 in FIG. 7) during which the blanking controlsignal is active may be relatively long, such that the voltage detectingcircuit can precisely detect feedback voltage signal V_(ZCS) by avoidingthe fluctuation of the feedback voltage signal.

In particular embodiments, the system can detect the feedback voltageaccording to practical applications rather than being affected byenvironmental or load interference, by controlling the length of theblanking time based on the peak current of the main switch. In addition,the control of the peak current may not be limited by the operationmodes, and the system may operate in a peak current control mode or aconstant on time control mode, or other appropriate operating modes.Thus, controlling the blanking time according in certain embodiments canbe applied to primary controlled flyback converters in any suitableapplications with high detection accuracy.

In particular embodiments, in a method for providing an adaptiveblanking time to a flyback converter, a main power stage of the mainpower switch can include a transformer having of a primary winding and asecondary winding, a main switch coupled to the primary winding, arectifying diode being coupled to the secondary winding. The method caninclude obtaining a blanking control signal according to a peak currentinformation of the main switch. The method can also include generating asampling time control signal according to the blanking control signaland the feedback voltage signal. The method can also include receivingthe feedback voltage signal and the sampling time control signal, anddetermining the time of detecting the feedback voltage signal accordingto the sampling time control signal to obtain a detection signal forcontrolling the main switch.

For example, the feedback voltage signal can represent an output voltageat the secondary side. When the blanking control signal is active, thefeedback voltage signal may not be detected, and the period during whichthe blanking control signal is active is adjustable along with the peakcurrent. Generation of the blanking control signal can include charginga capacitor by a current source, to generate a voltage signal across thecapacitor, and comparing the voltage signal against the referencesignal, to generate the blanking control signal. The current source cangenerate a current signal according to the peak current information ofthe main switch.

Generation of the sampling time control signal can include receiving thefeedback voltage signal and the blanking control signal, andtransmitting the feedback voltage signal to a holding circuit directlywhen the blanking control signal is active, and transmitting thefeedback voltage signal to the holding circuit through a delay circuitwhen the blanking control signal is inactive. The holding circuit cangenerate a holding signal based on the feedback voltage signal.Generation of the sampling time control signal can also includereceiving the feedback voltage signal, and obtaining a voltage signalthat is greater than the feedback voltage signal by differenceoperation, receiving the voltage signal and the holding signal, andgenerating the sampling time control signal.

The embodiments were chosen and described in order to best explain theprinciples of the invention and its practical applications, to therebyenable others skilled in the art to best utilize the invention andvarious embodiments with modifications as are suited to particularuse(s) contemplated. It is intended that the scope of the invention bedefined by the claims appended hereto and their equivalents.

What is claimed is:
 1. A control circuit, comprising: a) a blanking timecontrol circuit configured to generate a blanking control signalaccording to a peak current of a main switch of a power stage circuit ofa flyback converter, wherein said power stage circuit comprises atransformer having primary and secondary windings, said main switchcoupled to said primary winding, and a rectifying diode coupled to saidsecondary winding; b) a sampling time control circuit configured togenerate a sampling time control signal according to said blankingcontrol signal and a feedback voltage signal, wherein said feedbackvoltage signal represents an output voltage signal at the secondary sideof said transformer; c) a voltage detection circuit configured toreceive said feedback voltage signal and said sampling time controlsignal, and to determine the time of detecting said feedback voltagesignal according to said sampling time control signal in order to obtaina detection signal for controlling said main switch, wherein saidvoltage detection circuit stops detecting said feedback voltage signalwhen said blanking control signal is active, and the period during whichsaid blanking control signal is active is adjustable along with saidpeak current; d) a blanking circuit configured to receive said feedbackvoltage signal and said blanking control signal; e) a holding circuitcoupled to an output terminal of said blanking circuit for receivingsaid feedback voltage signal and providing a holding signal, whereinsaid feedback voltage signal is directly transmitted to said holdingcircuit when said blanking control signal is active, and said feedbackvoltage signal is transmitted to said holding circuit through a delaycircuit when said blanking control signal is inactive; f) a voltagedifference circuit configured to receive said feedback voltage signal,and to generate a first voltage signal that is greater than saidfeedback voltage signal; and g) a comparison circuit configured toreceive said first voltage signal and said holding signal, and togenerate said sampling time control signal.
 2. The control circuit ofclaim 1, wherein: a) said blanking circuit comprises a first resistorand a first switch coupled in parallel to said first resistor, saidfirst resistor having a first terminal for receiving said feedbackvoltage signal, and a second terminal coupled to said holding circuit,wherein said first switch is controlled by said blanking control signal;b) said holding circuit comprises a first capacitor having a firstterminal coupled to a second terminal of said first resistor, and asecond terminal that is grounded, wherein a voltage across said firstcapacitor is configured as said holding signal, and said first resistorand said first capacitor form said delay circuit; c) said voltagedifference circuit comprises a first voltage source having a negativepole for receiving said feedback voltage signal, and a positive polecoupled to said first comparison circuit; and d) said comparison circuitcomprises a first comparator having a non-inverting input for receivingsaid holding signal, and an inverting input for receiving said firstvoltage signal, wherein said sampling time control signal is activatedwhen said holding signal is greater than said first voltage signal.
 3. Acontrol circuit, comprising: a) a blanking time control circuitconfigured to generate a blanking control signal according to a peakcurrent of a main switch of a power stage circuit of a flybackconverter, wherein said power stage circuit comprises a transformerhaving primary and secondary windings, said main switch coupled to saidprimary winding, and a rectifying diode coupled to said secondarywinding; b) a sampling time control circuit configured to generate asampling time control signal according to said blanking control signaland a feedback voltage signal, wherein said feedback voltage signalrepresents an output voltage signal at the secondary side of saidtransformer; c) a voltage detection circuit configured to receive saidfeedback voltage signal and said sampling time control signal, and todetermine the time of detecting said feedback voltage signal accordingto said sampling time control signal in order to obtain a detectionsignal for controlling said main switch, wherein said voltage detectioncircuit stops detecting said feedback voltage signal when said blankingcontrol signal is active, and the period during which said blankingcontrol signal is active is adjustable along with said peak current; d)a charge and discharge circuit having a first current source, a secondswitch and a second capacitor, wherein said first current sourcegenerates a first current signal according to said peak current of saidmain switch; e) said second switch and said second capacitor beingcoupled in parallel, wherein said first current source charges saidsecond capacitor, and a voltage across said second capacitor isconfigured as a second voltage signal; and f) a comparison circuitcomprising a second comparator configured to compare said second voltagesignal against a first reference signal, and to generate said blankingcontrol signal.
 4. The control circuit of claim 3, wherein said blankingtime control circuit further comprises a second current source coupledin parallel to said first current source, wherein said second currentsource has a fixed value that is used to charge said second capacitor.5. A method, comprising: a) generating a blanking control signalaccording to a peak current of a main switch of a power stage circuit ofa flyback converter, wherein said power stage circuit comprises atransformer having primary and secondary windings, said main switchcoupled to said primary winding, and a rectifying diode coupled to saidsecondary winding; b) generating a sampling time control signalaccording to said blanking control signal and a feedback voltage signal;c) receiving said feedback voltage signal and said sampling time controlsignal, and determining the time of detecting said feedback voltagesignal according to said sampling time control signal in order to obtaina detection signal for controlling said main switch; d) using saidfeedback voltage signal to represent an output voltage at the secondaryside of said transformer, wherein said voltage detection circuit stopsdetecting said feedback voltage signal when said blanking control signalis active, and the period during which said blanking control signal isactive is adjustable along with said peak current; e) charging acapacitor by a current source to generate a voltage signal across saidcapacitor; and f) comparing said voltage signal against a referencesignal to generate said blanking control signal, wherein said currentsource is configured to generate a current signal according to said peakcurrent of said main switch.
 6. A method, comprising: a) generating ablanking control signal according to a peak current of a main switch ofa power stage circuit of a flyback converter, wherein said power stagecircuit comprises a transformer having primary and secondary windings,said main switch coupled to said primary winding, and a rectifying diodecoupled to said secondary winding; b) generating a sampling time controlsignal according to said blanking control signal and a feedback voltagesignal; c) receiving said feedback voltage signal and said sampling timecontrol signal, and determining the time of detecting said feedbackvoltage signal according to said sampling time control signal in orderto obtain a detection signal for controlling said main switch; d) usingsaid feedback voltage signal to represent an output voltage at thesecondary side of said transformer, wherein said voltage detectioncircuit stops detecting said feedback voltage signal when said blankingcontrol signal is active, and the period during which said blankingcontrol signal is active is adjustable along with said peak current e)receiving said feedback voltage signal and said blanking control signal;f) directly transmitting said feedback voltage signal to a holdingcircuit when said blanking control signal is active; g) transmittingsaid feedback voltage signal to said holding circuit through a delaycircuit when said blanking control signal is inactive, and providing bysaid holding circuit a holding signal based on said feedback voltagesignal; h) receiving said feedback voltage signal, and generating avoltage signal that is greater than said feedback voltage signal; and i)generating said sampling time control signal based on said voltagesignal and said holding signal.